In a conventional electrically driven power steering system for a vehicle, a vehicle speed and a steering torque generated at a steering shaft are detected, and a target driving current to a power assist electric motor is calculated based on the detected vehicle speed and the steering torque. Furthermore, a variation between the target driving current and an actual electric current flowing through the electric motor is detected, and thereby the electric motor is feedback controlled in such a way that the above variation becomes zero. As a result, an assist torque for the steering device is obtained in accordance with the vehicle speed and the steering torque. A brushless motor is generally used as the power assist electric motor, so that a possible problem occurring from brushes is avoided and an electrically driven power steering system of a high reliability is obtained.
In the above described conventional system, however, when a detecting error is generated at a current detecting device for detecting an electric current to the brushless motor, a torque ripple will be generated at the brushless motor, which would cause an unpleasant feeling in a steering operation. Accordingly, it has been necessary to correct the detecting error by the current detecting device.
In Japanese Patent Publication No. 2002-29432, for example, an electrical power steering system is disclosed, in which a comfortable steering feeling can be obtained by reducing the torque ripple at the power assist electric motor. Namely, an offset amount is corrected when the offset amount for the current detecting device has been changed due to a temperature variation.
An offset amount, however, depends on a temperature characteristic of each element of the current detecting device, such as an operational amplifier. In some of them, the offset amount increases as the temperature rises, or in some of them, the offset amount decreases as the temperature rises. Furthermore, in some of them, the offset amount increases when the temperature increases or decreases, namely the offset amount has a U-shape characteristic with respect to the temperature change.
In the above described Japanese Patent Publication, however, the temperature characteristic of the current detecting element is not considered. Accordingly, when a permissible range of the offset amount is set in consideration of tolerance of the current detecting element for the temperature characteristic, such permissible range might become too wide to detect an abnormal condition, even when an abnormal condition has occurred at the current detecting portion.
For example, in the prior art, the permissible range of the offset amount is determined in the following manner. In the case that, in FIG. 4, a permissible range of an offset corrective amount (voltage) at a lower temperature T1 is a range V1, and the permissible range at a higher temperature T2 is a range V2 in consideration of the tolerance of the current detecting element, a total permissible range of the offset amount is finally determined as a range V3 covering the ranges V1 and V2. Accordingly, a range V1′ is out of the permissible range V1 at the temperature T1, while a range V2′ is out of the permissible range V2 at the temperature T2. Nevertheless, those ranges V1′ and V2′ are respectively considered as a range within the total permissible range V3. As a result, even if the offset voltage is in the range V1′ at the temperature T1 or in the range V2′ at the temperature T2, those conditions are not determined as the abnormal operating condition.
It can be possible to manufacture the current detecting devices, in which the current detecting elements having the same temperature characteristic (for example, the offset amount always increases as the temperature rises) are selected. It is, however, a problem in such a method, that it requires a larger amount of manufacturing manpower and time, or it requires a larger number of the elements than the actually necessary number for the current detecting devices in view of an extraction rate, or it may cause an increase of the manufacturing cost when the current detecting elements which can not be used for the current detecting devices are disposed or diverted to other uses.